Researchers
recently published preliminary results of the first data collected with modern
techniques documenting the tectonic and seismic regimes of eastern Turkey. Their
work may overturn conceptions of the tectonic setting of the region.

In a series of what will be eight papers in all, seismologists, led by Eric Sandvol
from the University of Missouri-Columbia, are laying out the mantle tomography
(the velocity that seismic waves travel through different parts of the Earth),
crustal structure and other characteristics of the Anatolian plateau in the eastern
part of Turkey. The region has been out of reach for geologists over the past
decade and a half, owing to civil unrest between the native Kurdish people and
the Turkish government.

A 29-station broadband array collected seismic
reflection and refraction data in eastern Turkey over almost two years, the first
extensive survey of its kind in the region. On this topographic pseudo-color scale
map, white represents the maximum elevation. Image courtesy of Eric Sandvol.

People were concerned for their safety in that region, Sandvol says.
Over the 15-year period of war, lack of fieldwork and the impossibility of deploying
modern equipment meant, he says, that in terms of the subsurface structure,
we really knew almost nothing.

Sandvol and his coworkers from Cornell University in Ithaca, N.Y., and Bogazici
University in Istanbul, Turkey, installed a network of 29 broadband stations for
their Eastern Turkey Seismic Experiment. The Incorporated Research Institutions
for Seismology, a university research consortium, loaned the equipment from its
PASSCAL portable seismometers program for almost two years of data collection,
recording arriving seismic waves.

The teams initial interpretations from the seismic array data provide the
beginnings of a reassessment of older regional models, plus confirmation of some
ideas. The analyses show that there is no crustal and mantle thickening from the
collision of Arabia with Eurasia, as projected by some earlier models. What
we found was a slightly thickened crust and no lithospheric mantle, Sandvol
says, in addition to very slow seismic wave speeds in the mantle.

Velocity of seismic waves serves as a proxy for determining the temperature, density
and other characteristics of the material through which the waves traveled. The
low speeds indicate high temperatures, even hotter than the mantle underlying
the spreading African continental rift zone. The big question, Sandvol
says, is why is that there?

So far, the researchers results seem to rule out the subduction of the Arabian
plate underneath the Eurasian plate. Sandvol says, what Arabia does is just
stop, abruptly ending underneath the overriding Eurasian plate, instead
of extending at a gentle slope beneath it for hundreds of miles. That hypothesis
correlates with the lack of earthquakes deeper than 20 kilometers, which are common
to deep subducting slabs of crust, as well as to the surprisingly high mantle
temperature. Sandvol speculates that such underthrusting processes might occur
later in a collision, as they do in the nearby Tibetan Plateau, which would indicate
that the Turkey collisions are very young.

The first two papers in the eastern Turkey seismic series were published online
Sept. 10 in Geophysical Review Letters, covering tomography and seismic
anisotropy  waves traveling in different directions due to variations in
the mantle  under the plateau. The next papers will cover crustal structures,
seismicity, magma generation, and attenuation of seismic waves in the mantle beneath
eastern Turkey. All eight papers are expected to be published in a collected volume
this December.

The papers are preliminary results, says Nafi Toksöz, a seismologist
at the Massachusetts Institute of Technology in Cambridge, who is familiar with
the work. Though some of the results are not surprising, Toksöz says, some
of the anisotropy results are new and quite interesting, but Im not
sure how they will fit with other phenomena in the region. The two published
papers also report potentially conflicting anisotropies for shear and compressional
waves, which, he says, will require a mechanism to explain the differences.

Though Toksöz says he would have welcomed more details on the researchers
methods of analysis, the project was one of the most successful PASSCAL
experiments ever carried out. The data entered the public domain on Aug.
31, and Toksöz says he expects new results that either support or challenge
these preliminary ones from other seismologists soon.